The charge radius of the neutron-rich 6He (t1/2 = 807 ms) nucleus was determined to a precision of 0.7% in a laser spectroscopic measurement at Argonne National Laboratory. The 6He atoms were produced via the 12C(7Li, 6He)13N reaction at the Argonne Tandem Linac Accelerator System (ATLAS), and were excited to the metastable states by an RFdriven discharge. The spectroscopy was performed on individual 6He atoms confined and cooled in a magneto-optical trap. The isotope shift between 6He and 4He on the 23S1−33P2
transition at 389 nm was measured to be 43194.772 ± 0.056 MHz. Based on this measurement and the atomic theory calculation, the root-mean-square charge radius of 6He was determined to be 2.054 ± 0.014 fm. Combined with the interaction radius from
scattering experiments, the neutron-halo structure of the 6He nucleus was confirmed model-independently for the first time. This measurement helps reveal the three-nucleon forces inside the nucleus, and the result agrees well with the ab initio quantum Monte
Carlo calculations using a modern nuclear potential. The result is also compared with values predicted by various cluster models.